Duplex stainless steel has a ferrite phase in addition to an austenite phase generally considered to be resistant to embrittlement fracture, so the toughness is generally inferior to that of austenitic stainless steel.
As a factor behind the drop in toughness, in addition to the amount of the ferrite phase, the size of the solidified structure of the ferrite phase also has an effect. That is, the toughness is generally improved the greater the fineness of the structure, but duplex stainless steel solidifies at the single ferrite phase. The solidified structure is generally comprised of a coarse ferrite phase and an austenite phase finely precipitating at the grain boundaries and in the grains, so particularly in cast products, thick-gauge plate products, etc., the effects of the coarse ferrite phase are carried over as is to the final products.
As the techniques for increasing the fineness of the solidified structure, the techniques of electromagnetically agitating the cast slab during casting, controlling the overheating degree ΔT of the casting temperature to be small, etc. have been known, but these methods have the problems that they require bulky facilities and induce porosity defects. As opposed to this, there is the technique of utilizing TiN as the solidification nuclei. While this problem is small, it is liable to invite a drop in the toughness due to the introduction of nonmetallic inclusions, so it is necessary to study in detail the effects of increased fineness of the solidified structure and the harm due to the introduction of nonmetallic inclusions.
The inventors discloses the method of utilization of the action of nuclei of TiN on the δ iron in Japanese Patent No. 3624732, Japanese Patent No. 3624804, Japanese Patent No. 3446667, Japanese Patent No. 3458831, Japanese Patent Publication (A) No. 2002-69592, Japanese Patent Publication (A) No. 2006-117991, and Japanese Patent Publication (A) No. 1-100248.
Here, the first four patents relates to ferritic stainless steel, the next two patents relate to austenitic stainless steel having a high δ ferrite, and the final one patent relates to duplex stainless steel.
Among these, in particular the patents of Japanese Patent Publication (A) No. 2002-69592 and Japanese Patent Publication (A) No. 1-1002482 relate to inventions covering duplex stainless steel similar to the present invention, but all of these aim at improvement of the hot workability. Toughness is not considered at all.
Further, the first four patents relating to the ferritic stainless steel aim at improvement of the cold workability and toughness, but do not clearly give quantitative values relating to duplex stainless steel.
In the final analysis, there is no document clearly showing a realistic technique, for duplex stainless steel, for improving the toughness of cast steel and thick-gauge plate products aimed at by the present inventors.